This invention is concerned with a plunger mechanism for use in forming a gob of molten glass into a parison in a glassware forming machine and with methods of forming a gob of molten glass into a parison.
In forming glassware containers, a gob of molten glass is first formed into a parison and the parison is transferred to a mould in which it is blown into the shape of a container. Gobs of molten glass are formed into parisons either by a pressing operation or by a blowing operation and both such operations involve the use of a plunger mechanism. Conventional plunger mechanisms comprise a piston and cylinder assembly operable by the introduction of air under pressure into the cylinder to move the piston, a piston rod projecting from the piston, and means for mounting a plunger on the piston rod outside the cylinder so that the plunger moves with the piston. In most cases, the mechanism also comprises means for moving the piston into an intermediate position between its end positions, although, in some cases, this can be dispensed with. Where the parison is formed by a pressing operation, the plunger is, in most cases, moved from an out-of-the-way position thereof into an intermediate, loading, position in which the plunger projects into a mould cavity and a gob of molten glass is introduced into the mould cavity so that it rests on the plunger. The plunger is then moved from its intermediate position into an operative, pressing, position in which the plunger presses the glass against the walls of the cavity which are in the shape of the required parison. The plunger is then moved from its operative position to its out-of-the-way position to allow removal of the parison from the mould cavity. Where the parison is formed by a blowing operation, the plunger is, in most cases, moved from an out-of-the-way position thereof to an operative, loading, position in which the plunger projects into a mould cavity so that a gob of molten glass introduced into the mould cavity rests on the plunger. While the plunger is in its operative position, air is blown into the mould to cause the gob to settle on the plunger. The plunger is then moved from its operative position to an intermediate, counter-blowing, position thereof thereby creating a cavity in the gob into which air is blown to cause the gob to expand and the glass to be pressed against the walls of the cavity to form a parison. The plunger is then moved from its intermediate position to its out-of-the-way position to allow removal of the parison from the mould.
In conventional plunger mechanisms operated pneumatically, the time at which the plunger leaves its various positions is controlled by electronic or other timing means which controls the operation of valves which supply air under pressure to the cylinder of the plunger mechanism. However, there is no control over the time of arrival of the plunger in its various positions nor over the position or velocity against time curve followed by the plunger during its various movements.
It is an object of the present invention to provide a plunger mechanism for use in forming a gob of molten glass into a parison in which the position or velocity against time curve for the movement of the plunger is predetermined.